“…Bound states corresponding to positive eigenvalues have been realized also experimentally [6]. Since this initial example, Neumann-Wigner type potentials have attracted much attention [35,38,14,27,5,9,16,4,3,25,26,37]. In particular, the set of embedded eigenvalues is not necessarily a small set, Simon has shown that examples can be constructed for which there is a dense set of positive eigenvalues [36], see also [29,31,33].…”